1. Field of the Invention
The present invention relates to a semiconductor memory device and particularly to a structure of a bipolar RAM (Random Access Memory) having superior immunity to soft errors.
2. Description of the Prior Art
FIG. 1 shows a schematic diagram of a conventional bipolar RAM which is generally used.
Referring to FIG. 1, the bipolar RAM comprises a memory cell array MA having memory cells Cij (i=0, . . . n-1, j=0, . . . m-1) arranged in a matrix of n rows and m columns. The memory cell Cij is a Schottky clamped type (non-saturating type) and comprises two cross-coupled multiemitter transistors Q1 and Q2. The first emitter of each of the transistors Q1 and Q2 is connected to the bit line Bi and to the bit line Bi, respectively, while the second emitters are both connected to a hold line (negative word line) Hi. The collector of each of the transistors Q1 and Q2 is connected to the word line Wi through resistances R1 and R2, respectively. Schottky barrier diodes SBD1 and SBD2 are provided between the word line Wi and the collectors of the transistors Q1 and Q2. Having the above described structure, one memory cell forms a flip-flop.
In the row address system, there are provided as the peripheral circuit of the bipolar RAM, a row address buffer RA which receives external row address signals x.sub.0 .about.x.sub.l-1 to generate complementary internal address signals x.sub.0, x.sub.0, . . . x.sub.l-1, x.sub.l-1, an X decoder XD which decodes the internal row address signals from the row address buffer RA to generate address decoded signals V.sub.x0 .about.V.sub.x,n-1 to select one word line, and word drivers WD.sub.0 .about.WD.sub.n-1 which respond to the signal from the X decoder XD to select one word line and rise the potential of the selected word line. The word drivers WD.sub.0 .about.WD.sub.n-1 are provided in correspondence with the word lines.
The column address system comprises a column address buffer CA which generates complementary internal address signals Y.sub.0, Y.sub.0 .about.Y.sub.k-1, Y.sub.k-1 from external column address signals Y.sub.0 .about.Y.sub.k-1, a Y decoder YD which decodes the internal address signals from the column address buffer CA to select a pair of bit lines and bit line drivers BD.sub.0 .about.BD.sub.m-1 which connects the pair of bit lines to the data bit lines DB and DB in response to the output of the Y decoder YD. The bit line drivers BD.sub.0 .about.BD.sub.m-1 are provided in correspondence with the bit line pairs.
The data input/output system comprises current switch transistors Q.sub.0, Q.sub.0 '.about.Q.sub.m-1, Q.sub.m-1 ' provided at each bit line, a sense amplifier SA for detecting the collector current of the transistors connected to the selected bit line pairs and outputs a signal in response to the value of the collector current and, a write buffer WA which is activated in response to a signal WE identifying the operation modes of writing/reading to apply a voltage value corresponding to the writing data D.sub.in to the base of each current switch transistor. Each emitter of the switch transistors Q.sub.0, Q.sub.0 '.about.Q.sub.m, Q.sub.m ' is connected to the respective bit line. Each peripheral circuit is formed using bipolar transistors.
FIG. 2 shows the structure of a main portion of the memory portion of the bipolar RAM shown in FIG. 1. FIG. 2 shows memory cells C.sub.O0 and C.sub.O1. Each of multiemitter transistor Q1, Q2 of the memory cells comprises two transistors QC1, QH1 and QC2, QH2, respectively.
The potentials at the collectors of the transistor elements QC1, QH1, QC2 and QH2 are clamped by the Schottky barrier diodes SBD1 and SBD2, so that the transistor elements do not enter into the saturation state. For example, the memory cell C.sub.O0 is located between a word line W.sub.0 and a hold line H.sub.0 through which a constant current I.sub.H flows and between a pair of bit lines B.sub.0 and B.sub.0. The word line W.sub.0 is selected by a word driver WD.sub.0 and the bit line pair B.sub.0 and B0 are selected by a bit driver BD.sub.0 (not shown) so that the memory cell C.sub.O0 can be selected. However, it is also possible to select the memory cell C.sub.O0, even if only the word line W.sub.0 and all the bit line pairs are selected. The sense amplifier SA detects the data stored in the memory cells during the read mode. The write amplifier WA produces output potential V.sub.D and V.sub.D whose values are different when the potential of a write-enable signal WE is low, which means the write mode, and whose values are the same when the potential of the write-enable signal WE is high, which means the read mode.
The operation of the memory cell array of FIG. 1 will be explained by using FIG. 2, which is the main portion of FIG. 1. The access operation with regard to the memory cell C.sub.O0 (FIG. 1) is shown by the progression of the following steps.
Step 1: Non-selected state;
Step 2: The read mode in the selected state;
Step 3: The write mode in the selected state;
Step 4: The read mode in the selected state; and
Step 5: Non-selected state.
In step 1, it is assumed that the transistor elements QH1 and QH2 of the memory cell C.sub.O0 which form a flip-flop are conductive and non-conductive respectively. In addition, the transistor elements QC1 and QC2 of the memory cell C.sub.O0 are non-conductive, since the cell C.sub.O0 is non-selected. Further, the potential V.sub.D and V.sub.D of the write amplifier WA (FIG. 1) are the same, which means the read mode. The collector potential V.sub.BS is represented as follows. EQU V.sub.BS =V.sub.W0 -R.times.I.sub.BI
where R is the resistance of the resistor R2 and I.sub.BI is the base current of the transistor element QH1. In this case, the value R.times.I.sub.BI is very small so that the Schottky barrier diode SBD2 cuts off. Contrary to this, the collector voltage VCS is represented as follows. EQU V.sub.CS =V.sub.W0 -V.sub.BE
where V.sub.BE is the forward voltage of the Schottky barrier diode SBD1. In this case, the collector current I.sub.CI is relatively large so that the Schottky barrier diode SBD1 is conductive.
In step 2, the potential V.sub.W0 of the word line W.sub.0 is raised so that the collector potential V.sub.BS and V.sub.CS are also raised, while the potential V.sub.W1 of the word line W.sub.1 is lowered. The potentials V.sub.D and V.sub.D, which are the same, are preset to be between the potentials V.sub.BS and V.sub.CS. Preferably, the potentials V.sub.D and V.sub.D equal (V.sub.BS +V.sub.CS)/2. In this case, since the transistor element QC1, to whose base is applied the potential V.sub.BS, and a transistor Q.sub.0, to whose base is applied the potential V.sub.D which is lower than the potential V.sub.BS form a current switch, the transistor element QC1 conducts and the transistor Q.sub.0 remains non-conductive. Similarly, since the transistor elements QC2 and the transistor Q.sub.0 ' form a current switch, the transistor element QC2 remains non-conductive and the transistor Q.sub.0 ' conducts. As a result, currents indicated by arrows X1 and X2 flow through data bit lines DB and DB, respectively. Therefore, the currents I.sub.D and I.sub.D supplied to the sense amplifier SA are represented as follows. ##EQU1## In this case, the output data Dout of the sense amplifier SA is, for example, the logic "0".
In step 3, the potential V.sub.D is lowered which means the write mode for writing the logic "1". When the potential V.sub.D equals the potential V.sub.CS at time tl, the emitter currents of the transistor element QC2 and the transistor Q.sub.0 ' become equal (=1/2I.sub.B). Furthermore, when the potential V.sub.D becomes lower than the potential V.sub.CS, the emitter current of the transistor element QC2 becomes equal to I.sub.B and the emitter current of the transistor Q.sub.0 ' becomes zero. In this case, the difference in potential between the potential V.sub.CS and V.sub.D is, for example, higher than 200 mV. As a result, the potentials V.sub.BS and V.sub.CS become low and high, respectively, so that the state of the flip-flop is reversed. In this write mode, the potential V.sub.D is increased in order to spend up the write operation. However, it should be noted that the potential V.sub.D can remain at the same level as the read mode. In this case, the write speed becomes lower.
In step 4, the levels of the potentials V.sub.D and V.sub.D are the same. In this case, currents indicated by arrows Y1 and Y2 flow through the data bit lines DB and DB, respectively. Therefore, the currents I.sub.D and I.sub.D supplied to the sense amplifier SA are represented as follows. ##EQU2## In this case, the output data Dout of the sense amplifier SA is the logic "1".
In step 5, the potential V.sub.W0 of the word line W.sub.0 is lowered so that the collector potentials V.sub.BS and V.sub.CS are also lowered. However, the lowering of the potentials V.sub.BS and V.sub.CS takes a longer time than the lowering of the potential V.sub.W0. Contrary to this, the potential V.sub.W1 of the word line W.sub.1 is raised.
In steps 2 , 3 and 4 wherein the memory cell C.sub.O0 is selected and the memory cell C.sub.O1 is non-selected, both of the potentials V.sub.BN and V.sub.CN of the memory cell C.sub.O1 should always be lower than the potentials V.sub.D and V.sub.D so that the transistor elements QC1 and QC2 thereof are always non-conductive. Therefore, access into the memory cell C.sub.O1 is never effected.
In the bipolar RAM of the above described structure, some problems become serious which could conventionally be bypassed, as the bipolar RAM becomes more highly integrated. One of the problems is the soft errors induced by .alpha. particles.
The .alpha.-particle induced soft errors are induced by .alpha. particles emitted from radio active elements such as uranium and thorium which are slightly contained in the package material which holds the semiconductor chip in which the RAM is formed. The energy of the .alpha. particles emitted due to the .alpha. decay of the uranium and thorium centers mainly on 5 MeV and ranges to at most 9 MeV. Therefore, the .alpha. particles emitted from the package possibly have the energy of this level. The .alpha. particle of 5 MeV runs about 30 .mu.m in silicon, generating about 1.4.times.10.sup.4 electron-hole pairs. Specifically, the holes generated in the n type collector region flow to the substrate attracted by the field in the junction when they arrive at the collector.substrate junction. The electrons generated in the n type collector region and those electrons which are generated in the substrate and reached to the collector.substrate junction region and drawn into the collector region by the field in the junction, diffuse in the n type collector region. Consequently, there arises a current flow from the collector to the substrate. This lowers the collector potential of the off state transistor of the pair of transistors in the memory cell, causing the inversion of information in the memory cell. This phenomenon will be described with reference to the figures.
FIG. 3 shows a cross section of the structure of a multiemitter transistor in the memory cell of the bipolar RAM and the bipolar transistor contained in the peripheral circuit.
Referring to FIG. 3, the memory cell portion M and the peripheral circuit portion S are electrically separated from each other by a separating oxide film 8.
The bipolar transistor in the memory cell portion M is formed on a p.sup.- type semiconductor substrate 1 of, for example, silicon and comprises an n.sup.+ buried layer 2 for reducing the collector resistance and to provide electrical contact to the collector, an n.sup.- epitaxial layer 3 formed on the n.sup.+ type buried layer 2 and functions as a collector, an island of p.sup.+ type base diffusion region 4 formed on the n.sup.- epitaxial layer 3 and acts as a base and, n.sup.+ emitter diffused regions 5a and 5b formed on a prescribed region in the island of p.sup.+ type base diffusion region 4 and act as emitters. Aluminum wires 6a, 6b and 6d are provided in the n.sup.+ buried layer 2 and n.sup.+ emitter regions 5a and 5b,respectively. One of the aluminum wires 6b and 6d is connected to the bit line and the other is connected to a hold line for holding a constant current flow. An aluminum wire 6c is provided in the p.sup.+ type base diffusion region 4. The aluminum wire 6c is connected to a collector of the other memory transistor. The p.sup.+ type base diffusion region 4 and the n.sup.- epitaxial layer 3 are connected to the word line by the aluminum wire 6e. A resistance 10 is formed between the p.sup.+ type base diffusion region 4 and the aluminum wire 6e while a Schottky barrier diode 9 is formed between the n.sup.- epitaxial layer 3 and the aluminum wire 6e. Between each of the aluminum wirings 6a .about.6e there is provided an insulating film 7 for electrically separating the wire from each other.
The bipolar transistor of the peripheral circuit portion S comprises a p.sup.- type semiconductor substrate 1, an n.sup.+ type buried layer 2 for taking a collector electrode and for reducing the collector resistance, an n.sup.- type epitaxial layer 3 formed on the n.sup.+ type buried layer 2 to be the collector region, a p.sup.+ type diffusion layer 4 formed on the n.sup.- type epitaxial layer 3 to be a base region and an n.sup.+ type diffusion layer 5c formed as an island in the p.sup.+ base diffusion layer 4. Aluminum wires 6f, 6g and 6h are provided in the n.sup.+ type buried layer 2, p.sup.+ base diffusion layer 4 and the n.sup.+ type emitter diffusion layer 5c, respectively.
Referring to FIG. 3, in the transistor of the memory cell portion, a base-collector junction capacitance C.sub.PC is formed between the p.sup.+ type base diffusion layer 4 and the n.sup.- type epitaxial layer 3. Base-emitter junction capacitance C.sub.TE are formed between the p.sup.+ base diffusion layer 4 and the n.sup.+ type emitter diffusion layers 5a and 5b. A collector-substrate junction capacitance C.sub.TS is formed between the n.sup.- type epitaxial layer 3 (n.sup.+ buried layer) and the semiconductor substrate 1. The Schottky barrier diode 9 has a junction capacitance C.sub.SBD.
FIG. 4 shows an equivalent circuit of a conventional memory cell of a bipolar of RAM. FIG. 4 shows cross-coupled multiemitter transistors 11a and 11b, a resistance 10a provided between the collector of a transistor 11a and a word line 6, a Schottky barrier diode 9a, a resistance 10b provided between the collector of the transistor 11b and the word line 6, a Schottky barrier diode 9b and a hold line 12 for drawing a constant current from the memory cell. The junction capacitances 14a and 14b of the Schottky barrier diodes 9a and 9b, the base-collector junction capacitances 15a and 15b of the transistors 11a and 11b, the base-emitter junction capacitances 16a and 16b of the transistors 11a and 11b and the collector-substrate junction capacitances 17a and 17b of the transistors 11a and 11b are shown as the parasitic capacitances. The soft errors induced by .alpha. particles will be hereinafter described with reference to FIGS. 3 and 4.
Now, suppose that the multiemitter transistor 11a is off and the multiemitter transistor 11b is on. On this occasion, the potential of the collector of the transistor 11a , namely the potential of the node N, is "H". As described above, the potential V.sub.H of the node N is as follows: EQU V.sub.H =V.sub.W -R.I.sub.B
where V.sub.W is the potential of the word line 6, R is the resistance value of the resistance 10b and I.sub.B is the base current of the transistor 11a . Meanwhile the collector potential V.sub.L of the transistor 11b is as follows: EQU V.sub.L =V.sub.W -V.sub.BE,
where V.sub.BE is the forward direction potential drop of the Schottky barrier diode 9b.
As is apparent from FIG. 4, the capacitance C incidental to the node N will be EQU C=C.sub.TS +C.sub.SBD +4C.sub.TC +2C.sub.TE
By the radiation of .alpha. ray, the electron-hole pairs are generated in the collector-substrate junction in the memory cell. The charge thereof is denoted by .DELTA.Q and the change of the potential of the node N.DELTA.V will be .DELTA.Q/C.
The increase of the capacitance value C will minimize the above potential change .DELTA.V. The junction capacitances 14a and 14b of the capacitance value C.sub.SBD and the junction capacitances 15a and 15b of the capacitance value C.sub.TC are in parallel with the load resistances 10a and 10b of the memory cell, thereby serving as a speed-up capacitor at the time of information inversion. The capacitance value C.sub.TC of the junction capacitances 15a and 15b undergoes the effect of a factor by the multiple of 4 due to the Miller effect, so that the increase of the value C.sub.TC increases the stability against the .alpha. particle-inducing inversion of the information. Referring to FIG. 3, the junction capacitances 15a and 15b are the PN junction capacitance of the n.sup.- type epitaxial layer 3 and the p.sup.+ type base diffusion region 4. Accordingly, the capacitance value C.sub.TC will change according to the film thickness and the impurity concentration of the n.sup.- type epitaxial layer 3. Conventionally, the n.sup.- type epitaxial layer 3 was formed simultaneously with the memory cell portion and the peripheral circuit portion and it was controlled to have the same thickness and same impurity concentration.
In the conventional semiconductor memory device structured as above, if the thickness of the n.sup.- type epitaxial layer 3 is increased, the collector-base junction capacitance value C.sub.TC of the transistors in the memory cell portion and the peripheral circuit portion becomes small, enabling the high speed operation. However, due to the small capacitance value, the inversion of information in the memory cell induced by the .alpha. particles etc. is likely to occur. Likewise, if the impurity concentration of the n.sup.- type epitaxial layer 3 is decreased, the high speed operation becomes possible while the inversion of information easily occurs.
On the other hand, if the thickness of the n.sup.- epitaxial layer 3 is decreased, the capacitance value C.sub.TC increases and the inversion of information in the memory cell is not likely to occur. However, a high speed operation can not be expected. Likewise, if the concentration of the n.sup.- type epitaxial layer 3 is increased, the inversion of information in the memory cell is depressed, but a high speed operation can not be expected.
The mechanism of the .alpha. particles-induced soft errors in the bipolar RAM is described in the Transaction of IECE of Japan, 1980/2, VOL. J63-C No.2, K. Mitsusada et al. "Alpha-Particle-Induced Soft Errors in High Speed Bipolar RAM".
Bipolar RAMs devised for the soft errors are disclosed in Japanese Patent Laying-Open Gazette 4263/1981, Japanese Patent Laying-Open Gazette 196563/1982 and in the Japanese Patent Laying-Open Gazette 150266/1986.
In the first mentioned prior art, an ion implantation of As (arsenic) is carried out in the n.sup.+ type buried layer before forming the n.sup.+ type epitaxial layer and an n.sup.+ type buried layer is formed also in the n.sup.- epitaxial layer by the auto doping for forming the n.sup.- epitaxial layer, increasing the junction capacitance between the collector and the base. However, the thickness of the n.sup.- type epitaxial layer and the impurity concentration are not taken into consideration.
In the second mentioned prior art, the n.sup.+ type buried layer and the p.sup.+ base diffusion layer in the memory cell portion are in contact with each other to increase the collector-base junction capacitance of the memory cell transistor. In this prior art, the thickness of the n.sup.- type epitaxial layer and the impurity concentration are not taken into consideration.
In the third mentioned prior art, the n.sup.- type epitaxial layer in the memory cell portion are made thinner than the n.sup.- type epitaxial layer in the transistor of the peripheral circuit portion to increase the collector-base junction capacitance of the transistor in the memory cell portion. However, the relation between the impurity concentration of the n.sup.- type epitaxial layer in the memory cell portion and that of the n.sup.- type epitaxial layer in the peripheral circuit portion is not taken into consideration.